Polyethyleneterephtlate films (PET films) are used for a host of converting, printing, coating and metallizing applications. The thermal stability, dimensional stability, chemical resistance and relative high surface energy of polyethyleneterephtlate films are beneficial for typical end-use applications. For instance, polyethyleneterephtlate films are often used as coating bases for magnetic tape, thermal transfer ribbon, packaging materials, thermal lamination and many other web converted products.
Roll formed PET films are typically handled by converting processes involving roller trains. It is not uncommon for the PET films to slip or drag on the roller trains in the use of the PET films during converting operations, causing scratching on the film surfaces. In severe cases these scratches will render the PET films unusable or require the converted materials to be downgraded and sold as off-spec materials. Furthermore, in end-use applications it is not uncommon for the handling of converted PET films to result in scratching of the materials. Such applications in which surface scratches are not desirable include solar control window films, label uses, touch panel display, packaging and other graphic uses of PET film. Therefore, it is desirable to have PET films with a scratch resistance surface to improve the handling of the film in a conventional converting process.
Previously, prior art technology that attempted to render PET films more scratch and abrasion resistant involved either an inline process at the point of film manufacture, or and offline process done at the point of converting. In the case of inline processes utilized during film making, references in the prior art include descriptions of acrylic coating the film to improve abrasion resistance. Other efforts describe the use of specific inorganic or organic particles in the upper surfaces of the PET films. Further inline efforts include manufacturing PET films with increased crystallinity in the upper surfaces to improve the abrasion resistance of the materials. However, they do not render the PET film highly abrasion resistance. Therefore, extensive off-line converting solutions have been developed.
Previous solutions to render PET scratch resistant involve offline coating of PET films with a hardened surface coating. Such coating may consist of highly crosslinked acrylics, silicones, or inorganic chemistry like sol-gel coatings. For many applications, these offline-coated materials render the PET film highly scratch resistant. However, the relatively high cost of the offline converting of the PET to render the surface scratch resistant is a disadvantage. Therefore, there is a significant commercial need to manufacture PET films that are inherently scratch resistant via the utilization of a low cost and robust inline process.
Furthermore, many applications for PET films take advantage of the surface printability of the PET material. Labels, packaging materials, graphics and many other applications take advantage of this. However, many prior solutions to hardcoating the PET film to improve the abrasion resistance often render the PET film surface unprintable. Therefore, there is a commerical need for a low cost, scratch resistant polyester film with improved print receptivity.
U.S. Pat. No. 5,415,942 describes a biaxially oriented polyethylene terephthlate film having an abrasion resistant surface prepared by crosslinking an acrylic emulsion.
U.S. Pat. No. 4,801,640 describes a biaxially oriented polyester film having an abrasion resistant surface. The surface of the polyester film is made abrasion resistant via the inclusion of crosslinked organic particles and containing a crystallization nucleating agent.
U.S. Pat. No. 4,751,139 describes abrasion resistant polyester film prepared via processing conditions such that greater surface crystallization and thus better abrasion resistance is achieved.
U.S. Pat. No. 4,310,600 describes the construction of an abrasion resistant polyester film via surface coating the film with a silicone coating containing colloidal silica. Furthermore, the coating contains unsaturated acrylate monomers and/or oligomers. Bookxe2x80x94xe2x80x9c50 Years of Amino Coating Resinsxe2x80x9d American Cyanamid Company, 1986.
This invention relates to an abrasion resistant coated film including a polyester film coated with a highly crosslinkable melamine coating with a crosslinking density of greater than about 50% and less than about 100%, wherein the coating is cured after transverse stretching and heat setting the film.
This invention also relates to a melamine coating having a crosslinking density between about 50% and about 100% containing between about 0.01% by weight and about 1.0% by weight of a colloidal particle selected from the group consisting of silica, TiO2, graphite and blends thereof.
This invention further relates to a method of producing an abrasion resistant coated film including stretching a polyester film in the machine direction to form a uniaxially oriented film, applying a coating of highly crosslinkable melamine with a crosslinking density between about 50% and about 100% onto the uniaxially oriented film, heating and stretching the coated film in the transverse direction and curing the coating on the coated film.
We discovered a method to render PET films scratch resistant by use of a highly crosslinked inline coating. As is well known in the art, stretching a crosslinked coating is difficult and often renders the coating cracked. At higher draw ratios the physical integrity of the coating will be compromised. For instance, a cracked surface coating will result in an increased haze and reduce the applications for the scratch resistant materials. However, we discovered a method to produce scratch resistant films that overcome the previous limitations of cracking of a highly crosslinked coating. Such a discovery results in a clear surface coating that gives the polyester film a highly scratch resistant surface.
In our discovery, the coating is applied between the first and second stretching operations in the PET film manufacturing process. The coating is applied via any method convenient and well known in the art, such as via Mayer rod coating, Gravure coating, roll coating, die coating or other application and doctoring processes. As the coating enters the stenter, or stretching and heat setting ovens, the coating is dried to at least substantially completely remove water. After the water is removed, the coated PET film is then transversely stretched by about a ratio of between about 3 and about 4. Hence, the coating is also stretched the same amount as the carrier PET film.
During the drying phase, curing of the coating is accomplished. Curing of a highly crosslinked coating results in stretching a crosslinked solid by about a ratio of between about 3 and about 4. Such a process is physically difficult for most crosslinked coatings due to the limited extensibility of the materials. Therefore, we discovered a highly crosslinked coating that is not completely cured until the material enters the heat setting zone of the stenter. At this point, the coating is fully cured to produce a highly scratch resistant surface. Fully cured is an indication of the cross-link density of the film. We discovered that only highly crosslinked materials render useful properties. Such materials have a crosslink density of greater than about 50% of the reactive functional groups.